country, ponds surrounded by aquatic grass, hollow basins filled with
reeds, or woods with thick undergrowth, they must be carefully routed
out and searched; for these are places where men in ambush or insidious
spies are likely to be lurking.
[Chang Yu has the note: "We must also be on our guard against traitors
who may lie in close covert, secretly spying out our weaknesses and
overhearing our instructions."]